Interest in free space optical (FSO) communications continues to grow as the demands for higher bandwidth and faster data rates continue to increase. Generally speaking, an FSO system includes a pair of optical transceivers spaced apart from one another that transmit information back and forth using optical (e.g., laser) transmissions. Optical transmissions provide a wider bandwidth than other wireless communications mediums, such as RF frequency signals. Moreover, optical signals can generally be more focused than RF signals, and are thus more difficult to intercept and less likely to cause interference with other transmissions.
Despite the advantages of FSO systems, one drawback they have with respect to RF transmission systems is that the optical transceivers have to be precisely aligned with one another to operate properly. This may be difficult to do when the optical transceivers are spaced a significant distance from one another, such as a few miles or more. Moreover, in a typical FSO system, the optical transceivers are fixed in place once aligned with one another. As such, they cannot be easily re-directed to communicate with other transceivers as can a directional RF antenna, for example.
Various attempts have been made in the prior art to provide more ready alignment of FSO optical transceivers. U.S. Pat. No. 6,381,055 to Javitt et al. discloses a system for aligning optical transceivers in which reflectors are positioned near each transceiver. The reflectors are used to calibrate, align, and/or re-align the transceivers by reflecting a beam of light back toward its source. A control unit controls the motion of the transceiver assembly, either locally or remotely. In particular, Javitt et al. discloses that the transceiver assemblies may be rotated, moved up or down, or that an elevation angle thereof may be adjusted.
Another potential drawback of FSO systems is that the optical wavefronts being transmitted from the transceivers are subject to atmospheric distortion, such as from heat rising from the earth, etc. To compensate for such distortion, some systems have begun to use adaptive optics which can restore a wavefront to its original shape. By way of example, such adaptive optics may include a deformable mirror that can be selectively deformed to reshape an optical wavefront.
One exemplary prior art deformable mirror is disclosed in U.S. Pat. No. 6,464,364 to Graves et al. The mirror is capable of controlled deformation by applying electrical voltages to electrode segments on the back of the mirror. Two plates of an electro-restrictive material, such as lead zirconate titanate (PZT) or lead magnesium niobate (PMN), are jointed together with at least one conductive layer sandwiched therebetween. One plate has an outer conductive layer and a mirrored surface on the outer conductive layer. The other plate has a pattern of a plurality of electrode segments on the outer surface. Each electrode segment has a separate electrical terminal for applying a variable electrical voltage thereto for separately transmitting a variable current through each electrode segment and through at least the other plate. This causes variable expansion of the plate to selectively deform that plate and, in turn, the deformable curvature mirror.
While adaptive optics do provide enhanced reliability, they are typically fairly expensive to implement. Moreover, a relatively large power supply on the order of several hundred volts may be required for such devices. Such high voltages are needed to provide deformation of the mirror over its entire deformation range. As such, with the requisite power and control circuitry required for the deformable mirror, an FSO system incorporating such technology can be fairly large and cumbersome to move and set up.
Moreover, to provide optical communications over a wide range of distances, most optical transceivers include a telescopic zoom lens device that has a fairly large aspect ratio. Yet, this may also increase the cost of the transceiver assembly, in addition to increasing its overall size.